The ringers or audio devices of conventional telephone subsets are designed for audibly responding to ringing signals applied thereto from along typical tip and ring leads of subscriber lines. Generally, this a.c. ringing signal will be asserted intermittently in about one second intervals at a frequency of between about 16 to 60 Hertz and at a voltage in the range of about 50-110 v. For the most part, telephone ringers are of the electromechanical type, an inductive component actuating a bell clapper for the duration of the applied ringing signal.
Because of their inherent bulk and relatively large power consumption characteristics, the electromechanical ringers are being replaced in telephonic product designs by piezoelectric driven diaphragms or small loudspeakers operating in conjunction with solid state integrated ring logic circuits. While the piezoelectric ringers promise the advantages of low power consumption and small size, their implemention as practical components has been seen to encounter a variety of technical difficulties.
Inasmuch as telephone system line networks extend within a broad range of environments, a variety of spurious signals are developed upon them and it becomes necessary for the ringing circuits to distinguish a valid ringing signal from spuriously derived pulses. Some of these spurious signals exhibit quite high voltages, lightning generated signals reaching 1,000 volts and higher. Other signals which require rejection can be generated at the subscriber's telephone itself. For instance, an audible output referred to in the industry as "bell-tapping" may be derived during the course of the dialing operation. Inasmuch as a significant amount of current flows during dialing, the long lines associated with the telephone network exhibit an inductance to an extent wherein inductive spikes may be generated as current is terminated which may actuate a ringer circuit. Accordingly, the logic circuits associated with ringers must be capable additionally of distinguishing bell-tapping phenomena and rejecting such signals. In effect, the ringer circuits must be capable of evaluating an incoming signal in a manner wherein its periodic nature is recognized, following which, the circuit must accept the ring signal. Preferably, the circuits will observe 2 to 4 cycles of the periodic ring signal in the process of detecting it.
Another aspect of the design of solid-state ringing devices is concerned with the quality of audio output achieved. In this respect, there resides a need to achieve a necessary level of volume or loudness. The energy available for driving the associated piezoelectric components should only be that derived from the ringing signal itself. Thus, impedance defining components must be minimized or otherwise effectively treated. In addition to achieving necessary loudness, it also is necessary to develop a sound quality which is pleasing to the consuming public. It is desirable in this regard to excite the piezoelectric element of the sound producing diaphragm at two frequencies at a distinct warble rate. Improper selection of the latter warble rate effects the generation of a raucous sound, while the excitation frequencies themselves should be carefully chosen for sound quality. Generally, the two frequencies should be somewhat close to achieve this quality.